On Atomic and Molecular Structure (1922–1925)

Abstract

In the years from 1925 to 1927 two fundamental insights were established in physics. One was the recognition of the degree to which the concepts of classical physics had to be restricted in the atomic domain because of the indivisibility of the quantum of action h. The second was the observation that, in the case of elec­ trons as indistinguishable particles, Pauli's exclusion principle and Fermi's sta­ tistics applies, and that the electrons possess a spin of magnitude hi 4 n. These two insights belong to two different levels of the description of physical reality, to quantum mechanics and to the theory of elementary particles. The corre­ sponding phenomena were, however, closely intermeshed in the spectroscopic data. Accordingly, prior to 1925 quantum theory had to struggle with two un­ resolved difficulties, and it is thus understandable that, in the process, wrong paths were taken. This early phase of the history of quantum theory, which may be denoted by the title of A. Sommerfeld's book: Atomic Structure and Spectral Lines [1], was ushered by N. Bohr in 1913 with his theory of the hydrogen spectrum. The latter already contained the germ of the correspondence principle, according to which the frequencies and intensities of radiation emitted from atoms correspond to the frequencies and intensities derived from a classical model of the atom and become identical in the limit of large quantum numbers. Thus, the merging of atomic dynamics, as yet unknown, into the already familiar classical mechanics became conceivable. With these notions Sommerfeld and Bohr succeeded in explaining the basic pattern of spectral lines. The starting model consisted of an "external electron" moving in the electric field of the "atomic core", which